From the standviews of resource-saving and environmental health, an aqueous paint based on a water soluble or dispersible binder resin has been widely used. However, most of the surfaces to be coated with such a paint are more or less of hydrophobic nature and therefore, are inadequate for the heretofore proposed aqueous coating compositions because of poor adhesion between the coating and substrate surfaces. An aqueous paint containing a water soluble or dispersible resin has in general a problem of poor water resistance of the formed coating. In the case of the so-called high solid aqueous paint, water insoluble resin particles are included in such a paint for the improvement in sagging property of the coating. However, since a surface active agent is likewise present in the system to assist the desired dispersion stability of the resin particles, the presence of such material will give an additional cause for the decrease in water resistance of the formed coating. Furthermore, an aqueous paint has another problems of poor gloss and smoothness of the coating.
Recently, for the improvements in water resistance and appearance of coating and storage stability of coating composition, have been proposed a technique of dispersing in an aqueous paint fine particles of a solution of insoluble resin in an organic solvent which is immiscible with water (Japanese Patent Publication (unexamined) No. 181367/87); and a technique of using an aqueous paint containing a particular amount of particular F-containing random copolymer (Japanese Patent Publication (unexamined) No. 158766/87) and the like. In these techniques, a particular aqueous resin or water insoluble resin is selectively used, thereby obviating the use of conventional surfactant and preventing the decrease in water resistance of the coating, and water insoluble resin is compounded in an aqueous paint, thereby improving gloss and smoothness of the resulted coating. However, in the formed coating, there always co-exist water soluble or dispersible resin and water insoluble resin, and attainable adhesion properties of the formed coating towards substrate surfaces are not enough.
It is, therefore, an object of the invention to provide a novel method for forming, on hydrophobic surfaces of a substrate, an excellent coating in respects of adhesion, water resistance and appearance, by using an aqueous coating composition.